Typically, a transformer includes a housing which surrounds a core-and-coil unit and has a plurality of openings in which the bushings fit. One end of each bushing is connected to the core-and-coil unit usually near the top of the housing. The inside of the housing is usually filled with a fluid, such as transformer oil. The other end of each bushing which is located outside of the housing through one of the openings is coupled to another conductor, such as a power line. Typically, transformers used by power companies handle voltages ranging between about 15 kV to 765 kV and currents ranging between about 400 and 2000 amps.
One type of bushing is known as a bottom-connected bushing. In a bottom-connected bushing, the transformer windings of a core-and-coil unit are connected directly to the bottom of the conductor for the bushing in the transformer housing. The conductor is fixed within an insulating layer for the bushing.
Another type of bushing is known as a draw-lead bushing. In a draw-lead bushing, the transformer windings of the core-and-coil unit are connected to a flexible cable. The cable is not connected to the bottom of the bushing, but is pulled through a hollow metallic tube that is fixed within the insulating layer for the bushing. The electrical connection is made at the top of the bushing under the top terminal.
Yet another type of bushing is known as a draw-rod or split-conductor bushing. The draw-rod bushing is identical to the draw-lead bushing, except that a solid conductor is used instead of a flexible cable as the conductor. For ease of installation, in some bushings the solid conductor is split in the middle. These bushings are known as split-conductor bushings.
Draw-lead, draw-rod, and split-conductor bushings have limitations in their current carrying capacity because their size is limited by the inside diameter of the hollow metallic tube that is fixed within the insulating layer for the bushing. Typically, bottom-connected bushings can carry more current than draw-rod or split-conductor bushings and draw-rod or split-conductor bushings can carry more current than draw-lead bushings.
The current capacity of a particular bushing is also limited by the temperature rise of the conductor in the bushing. The temperature rise is the result of the heat produced by the resistive losses of the conductor and is greatly affected by the ability of the surrounding media to conduct heat away from the conductor.
One of the problems with bushings is their ability to effectively dissipate heat from the conductor. In the case of bottom connected bushings, the heat can be transferred fairly efficiently to the dielectric material surrounding the conductor. However in the case of draw-lead, draw-rod, and split-conductor bushings, the heat is not transferred as efficiently because the conductor is spaced from the metallic tube which surrounds the conductor by air or other substances. Another problem is the diameter of the conductor is limited by the inside diameter of the metallic tube. As a result, the temperature of the conductor increases, limiting the current carrying capacity of the bushing.
A problem unique to the split-conductor is the contact resistance at the interface between the halves of the split conductor. As a result, the current carrying capacity of the bushing is limited because of the heat produced at the interface.
Another problem with bushings is with their installation and maintenance. There are several instances when one or more bushings may need to be installed into the housing and connected to the core-and-coil unit. For example, when a transformer is initially installed, the transformer is often too bulky to transport to the installation site with the bushings attached. As a result, the bushings are removed and then reinstalled at the installation site. With existing transformers, over time one or more bushings may need to be replaced due to damage or deterioration. Currently, the design of the transformers and bushings makes the process of installing or replacing a bushing in a transformer an expensive and time consuming process.
As discussed above, in many existing designs for transformers the connection point between the bushing and the core-and-coil unit is adjacent the top of the housing of the transformer. Accordingly, to install one or more of these bottom-connected bushings, the transformer oil must be drained from the housing. Once the transformer oil is drained, then a worker must climb inside the housing and connect the bushing to the core-and-coil unit. This connection process is time consuming and expensive.
Additionally, this installation process exposes the interior of the housing to moisture. As a result, the inside of the housing and the core-and-coil unit must be thoroughly dried before the transformer oil is refilled into the housing. The drying process can take a substantial amount of time. By way of example, this drying process typically takes between about one and three days. Accordingly, like the connection process, this drying process is time consuming and expensive and delays the start of actual use of the transformer. Attempts to overcome or minimize these problems have met with limited success.
As discussed above, to improve the installation process, some transformers use draw-lead, draw-rod, and/or split conductor bushings. Basically, one end of the draw lead, draw rod, or split conductor is coupled to the core-and-coil unit near the bottom of the housing. The other end of the conductor is positioned to extend out from the opening in the housing. The tube of insulation for the bushing is simply placed over the conductor which extends up and out of the transformer. These bushings are relatively easy to install, but have other problems as discussed earlier.